Field of Invention
The present invention relates to a urea water supply system that supplies urea water to the exhaust emission for reduction of NOx using a selective reduction NOx catalyst provided in an exhaust passage of an internal combustion engine.
Description of the Related Art
A known configuration of an exhaust emission control device provides a selective reduction NOx catalyst (hereinafter simply referred to as “NOx catalyst”) that reduces NOx included in the exhaust emission discharged from an internal combustion engine by using ammonia as a reducing agent. A supply valve is provided in the upstream of the NOx catalyst to supply urea water to the exhaust emission, in order to control the adsorption amount of ammonia on the NOx catalyst to a suitable condition for reduction of NOx. Urea water used for producing ammonia causes deposition of urea after vaporization of its water content. This may adversely affect components involved in supply of urea water, for example, the supply valve and a pump for pressure-feeding urea water.
For example, Patent Literature 1 describes the adverse effects of the released urea on the pump. More specifically, Patent Literature 1 has noted that urea depositing by vaporization of water after a stop of operation of the pump is likely to enter the clearance between components of the pump and cause a trouble and provides a configuration of making a continuous flow of urea water in the pump even after a stop of operation of the pump, in order to suppress deposition of urea.